A researcher at Duke University in Durham, N.C., Drea has studied female spotted hyenas, which display a suite of traits typically described as masculine. Female hyenas dominate the species' rough-and-tumble social life and even grow penislike genitalia. Analyses of the animals' hormones have linked these traits to a system influenced by prenatal exposure in females to extra androgens, which are usually masculinizing hormones.

Now, Drea has turned her attention to the Strepsirrhini suborder of primates, which includes lemurs and their relatives. Females of many of these species likewise dominate social groups and develop enlarged genitalia that look male.

She monitored mothers' blood concentrations of hormones throughout more than a dozen pregnancies among ring-tailed lemurs (Lemur catta). The lemur moms' androgen concentrations were higher overall when they were carrying male fetuses. Yet, as they do in hyenas, the pregnant lemurs' androgens surged during critical periods when the female fetuses were developing their sex organs. That pattern isn't typical of mammals without dominant females.

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